1. Field of Invention
The present invention relates to a semiconductor laser element. More particularly, the present invention relates to a multiwavelength quantum dot laser element capable of simultaneously emitting lasers of various different wavelengths under the condition that an applied current is larger than a start-oscillation current.
2. Related Art
Recently, it is found in many researches that laser elements using quantum dot structure as light emitting region are characterized in having high quantum efficiency and low start-oscillation current, etc., such that the quantum dot semiconductor laser has developing potentials in industrial applications, such as fiber communication or optical data storage.
The conventional method of fabricating the quantum dot mainly adopts molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) and uses a self-assembly method to make the quantum dot self-assembled on a specific substrate surface. Recently, the self-assembled quantum dot can be applied in semiconductor amplifiers, superluminescent diodes, tunable wavelength semiconductor lasers, and multiwavelength semiconductor lasers, and so on.
For the application of the conventional multiwavelength semiconductor laser, for example, in Japanese Published Patent No. 2000340883, the material gain provided by more than three different quantum dot sizes in each of the quantum dot layers is utilized, and a super structure grating structure is used to control the laser gain with wavelength selectivity, in which each quantum dot can be grown by Stranski-Kranstanov (SK) or atomic layer epitaxy (ALE).
In addition, in Japanese Patents Publication No. 10112567 and No. 63213384, different quantum dot sizes are adjusted to generate multiwavelength lasing effect. As for the former, mainly, openings with different sizes are etched in the same layer or different layer structures, and quantum dot structures with various sizes are grown on the openings. As for the latter, quantum dot structures with various sizes are grown in different regions on an InP substrate, and current is injected by individual electrodes to obtain the simultaneous emitting of the multiwavelength laser.
However, as the growing condition of each of the quantum dot layers is the same, and each of the quantum dot layers grows the quantum dot structure through self-assembly, the difference of the quantum dot sizes is limited. In addition, the above-mentioned laser element structure further has worries in high start-oscillation current, high operating current, and poor stability, etc., so it is necessary to provide a multiwavelength semiconductor laser element capable of providing high stability, low start-oscillation current, and low operating current.